I notice that the parts lists for the XR-VCO (as shown on Scott's "Birth-of-a-SYnth" site) and the TH-102 ElCerrito (ref: Magic Smoke) are very close but not quite the same. Tim mentioned the TH102 PCBs in one of the XR-VCO threads here, but it looks like they're not quite interchangeable... So what's the difference between the two? Anyone????

Thanks, wmonk... if the suboctave is the primary difference I think I'll just go ahead with the XR-VCO circuit from Scott's page, as I've got suboctave generators on my MFOS Wave Freakers (as well as on a few other modules, if memory serves)... Can't wait to get started.

After your post I went to look at it in some more detail, since I have the XR-VCO on my breadboard right now.

In addition to the Suboctave outputs, there is also a PWM output that's not on the XR, with CV of the pulse, separate "rampoid" output, and a switch between audio/LFO frequencies (if I'm reading it right). That last bit would be easy enough to do yourself, btw, but I'm not sure about the others.

Magic Smoke has also added PTC resettable fuses for short circuit protection and diodes for reverse voltage protection that aren't in the original circuit.

I've opted to go ahead and get the el Cerrito, but then, I'm just starting out with modular and this is my first VCO, so I don't have the suboctaves or any other other bits.

Thanks for the update, elmegil, I guess there are more changes than meet the eye. I may go ahead and order an ElCerrito myself, as I already own a TH-101 Sheboygan so it would be nice to round out the set.

Just the same, I'm going to go ahead with the XR-VCO pretty much as is... I really wanted to do it to get some practice redrawing PCB layouts for engraving. Picked up a CNC engraver to do front panels so thought I would expand my horizons and get some practice with the software and equipment. Here's how the drawing for the PCB layout from the "Birth-fo-a Synth" site worked out... I'm pretty happy with it. I just noticed that I missed re-labeling C13... it's an electrolytic and needs to match the other ones... minor detail, all the hard work is done. BTW, I can probably make that audio/LFO range change myself, will look at the schematics later to see what's involved. Truth be told, I'm happy to implement it just as Thomas shows... if it was good enough for him, etc...

<marv>

ps... whereabouts in Chicago are you? I'm from there originally, still have lots of family & friends around. PM me if you'd rather not put it out in public.

Yup, they're basically the same, I just added some extra waveforms and PWM. Here's a summary from the El Cerrito Build Notes:
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Although the core of the VCO remains the same as the original Thomas Henry design, a few additional features were added. These features are highlighted on the schematic (enclosed in a double blue line) and include:
1) Added “Rampoid” waveform. This waveform is tapped off the timing capacitor, and is buffered by IC5a. The Rampoid sounds much like a traditional Sawtooth wave.
2) Added variable-width Pulse waveform. The Pulse wave is derived from the Tri / Sine wave, and will shift octaves just like the Tri / Sine wave does (although the octave shift will happen at different points, depending on where the PWM threshold is set). See the waveform graphics below for an illustration of how this happens as the Skew control voltage goes from 0 to 100%.
3) Added Sub-octave derived from the Pulse waveform. This waveform does not track the changing pulse width of the pulse wave, but it will shif t octaves as the Tri / Sine wave skews from a triangle to a 2x Saw.
4) Added Sub-octave derived from Square waveform.
5) Added Hi / Low switch to change timing capacitors. The TH-102 has a range of roughly 33,000:1, so these two settings will cover a very wide range of frequencies.
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The attached copy of the Build Notes shows the various waveforms, including the Rampoid and Skew waves. There's really nothing magical about the extra waves, and they could easily be added to the publicly posted XRVCO design. The main benefit to mine is that you don't have to etch your own PCB. Any questions?

As I haven't seen the TH102 schematic I can only imagine how most of these functions were implemented. I do like the idea of all that extra functionality, though, so check your email for my Paypal for a TH102 PCB. Thanks for making these happen.

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